1. Field of the invention
The invention concerns an input screen scintillator for an X-ray image intensifier tube. It also concerns the manufacturing process of this scintillator.
2. Description of the prior art
X-ray image intensifier tubes are well-known in the prior art. For example, these tubes are used to transform X-ray images into visible images for medical observation.
These tubes consist of an input screen, an optoelectronic device and an observation screen.
The input screen includes a scintillator which converts incident X photons into visible photons. These photons then strike a photocathode which is generally made of an alkaline antimonide. The photocathode is excited by the photons and generates a flow of electrons. The photocathode is not deposited directly on the scintillator but on a conductive underlayer which can reconstitute the charges of the photocathode material. This underlayer can for example be made of alumina, or of indium oxide or a mixture of these two substances.
The electron flow from the photocathode is then transmitted by a system of electron optics which focuses the electrons and sends them towards an observation screen consisting of a luminophore, which then emits visible light. This light can then be converted into television or cinema images, or into photographs.
The input screen scintillator is generally made of cesium iodide needles formed by vacuum evaporation on a substrate. The evaporation process can take place either on a cold or hot substrate. This substrate could preferably be an aluminium substrate. A cesium iodide layer usually 150 to 500 .mu.m thick is then deposited on it.
Cesium iodide deposits naturally as 5 to 10 .mu.m diameter needles. Its refractive index of 1.8 makes it behave like an optical fibre, and this tends to lessen the lateral diffusion of the light generated within it.
FIG. 1 is a schematic drawing showing an aluminium substrate 1 with several cesium iodide needles 2 on it. The aluminium substrate receives a flow of X photons symbolized by vertical arrows. Several examples of the paths along which the visible radiation created by the incident X photons travel within the cesium iodide needles are shown on this drawing. The normal traveling paths of this visible radiation, which are referenced 3, produce a luminous signal at the tips of the cesium iodide needles. However, a lateral diffusion of the light conveyed by the cesium iodide needles also occurs, as is shown by reference 4 on the drawing. This lateral diffusion tends to impair the tube resolution. The quality of the resolution depends on a correct channeling of the light by the cesium iodide needles, but also on the thickness of the cesium iodide layer: thicker layers tend to impair resolution. On the other hand, thicker cesium iodide layers also result in a better absorption of X-rays. A compromise must therefore be found between a sufficient X-ray absorption and a high resolution.
During the manufacturing process, the input screen must be subjected to a heat treatment which can also influence the tube resolution. This heat treatment occurs immediately after the cesium iodide has been vacuum evaporated. The treatment makes the screen luminescent, since the cesium iodide has been doped by sodium or thallium ions. It consists in heating the screen to a temperature of 340.degree. C. for about one hour in a desiccated air or nitrogen atmosphere.
During this heat treatment, which is an absolutely essential step, the scintillator needles coalesce and agglomerate, as shown by the schematic drawing on FIG. 2. This coalescence favors an increased lateral diffusion of light (as is shown by the dotted arrows referenced 4) which impairs the resolution.
In the prior art, it had been suggested to make the input screen scintillator by alternatively evaporating pure cesium iodide and cesium iodide doped with a refractory material to suppress coalescence during the heat treatment. The anticipated result was that needles made of alternate layers of pure cesium iodide and doped cesium iodide would not agglomerate during the heat treatment.
However, this solution failed to work effectively. Moreover, a structure of alternate layers of pure cesium iodide and doped cesium iodide does not at all prevent another serious problem, i.e, the lateral diffusion of light.
It was therefore proposed, as described in the U.S. Pat. No. 4,069,355 published on Jan. 17, 1978, to coat the cesium iodide needles with titania or gadolinium oxysulfide or lanthanum oxysulfide. The use of these deposited materials, which contain a metal, not in a metallic form, but in the form of an oxide or a compound, can partially solve the above-mentioned problems: it prevents needles from coalescing and slightly lowers the lateral diffusion of light, although this lower diffusion does not noticeably increase the scintillator's efficiency.
However, the problem of electrical conduction remains unsolved, even in the above-mentioned patent: any layer coating the needles should permit conduction while avoiding coalescence and the lateral diffusion of light. A good electrical conduction is necessary to increase the scintillator's efficiency by obtaining the same potentials in the coating layer of the needles, in the aluminium substrate on which the needles are formed and at the annular electrode to which the substrate is connected.
A first object of the invention, therefore, is to find a solution for these drawbacks by making a scintillator in which the cesium iodide needles are coated with a highly conductive material to prevent said needles to coalesce while sensibly decreasing the lateral diffusion of light. These goals can be achieved by choosing either a semiconductive material or a metal, to the exclusion of metallic oxides.